Pyrylium dye overcoating of pyrylium dye sensitized photoconductive elements

ABSTRACT

HIGH SPEED ELECTROPHOTOGRAPHIC ELEMENTS ARE PREPARED BY FORMING A PHOTOCONDUCTIVE COMPOSITION COMPRISING AN ORGANIC PHOTOCONDUCTOR SENSITIZED WITH A TWO-PHASE HETEROGENEOUS COMBINATION OF A SENSITIZING DYE AND A FILMFORMING HYDROPHOBIC POLYMER, COATING THIS COMPOSITION ONTO A CONDUCTING SUPPORT AND THEN OVERCOATING THE COMPOSITION WITH A SOLUTION OF A SENSITIZING DYE IN A VOLATILE HALOGENATED HYDROCARBON SOLVENT.

United States Patent US. Cl. 961.6 12 Claims ABSTRACT OF THE DISCLQSURE High speed electrophotographic elements are prepared by forming a photoconductive composition comprising an organic photoconductor sensitized with a two-phase heterogeneous combination of a sensitizing dye and a filmforming hydrophobic polymer, coating this composition onto a conducting support and then overcoating the composition with a solution of a sensitizing dye in a volatile halogenated hydrocarbon solvent.

This application is a continuation-in-part application based on U.S. application Ser. No. 708,805, filed Feb. 28, 1968, now abandoned.

This invention relates to electrophotography and to photoconductive elements and structures useful in electrophotography. In addition, this invention relates to methods for preparing electrophotographic elements.

Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, US. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, these processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well known in the art, can then be employed to produce a permanent record of the image.

One type of photoconductive insulating structure or element particularly useful in electrophotography utilizes a composition containing a photoconductive insulating material. A unitary electrophotographic element is generally produced in a multilayer type of structure by coating a layer of the photoconductive composition onto a film support previously overcoated with a layer of conducting material or the photoconductive composition may be coated directly onto a conducting support of metal or other suitable conducting material. Such photoconductive compositions have shown improved speed and/ or spectral response, as Well as other desired electrophotographic characteristics when one or more photosensitizing materials or addenda are incorporated into the photoconductive composition. Typical addenda of this latter type are disclosed in US. Pat. Nos. 3,250,615, 3,141,770 and 2,987,- 395. Generally photosensitizing addenda to photoconductive compositions are incorporated to effect a change in the sensitivity or speed of a particular photoconductor system and/ or a change in its spectral response characteristics. Such addenda can enhance the sensitivity of an element to radiation at a particular wavelength or to a broad range of wavelengths where desired. The mechanism of such sensitization is presently not fully understood. The phenomenon, however, is extremely useful. The importance of such effects is evidenced by the extensive search currently conducted by Workers in the art for compositions and compounds which are capable of photosensitizing photoconductive compositions in the manner described.

Usually the desirability of a change in electrophotographic properties is dictated by the end use contemplated for the photoconductive element. For example, in document copying applications the spectral electrophotographic response of the photoconductor should be capable of reproducing the wide range of colors which are normally encountered in such use. If the response of the photoconductor falls short of these design criteria, it is highly desirable if the spectral response of the composition can be altered by the addition of photosensitizing addenda to the composition. Likewise, various applications specifically require other characteristics such as the ability of the element to accept a high surface potential, and exhibit a low dark decay of electrical charge. It is also desirable for the photoconductive element to exhibit high speed as measured in an electrical speed or characteristic curve, a low residual potential after exposure, and resistance to fatigue. Sensitization of many photoconductive compositions by the addition of certain dyes selected from the large number of dyes presently known has hitherto been widely used to provide for the desired flexibility in the design of photoconductive elements in particular photoconductor-containing systems. At the present time, however, no photosensitizer addenda to photoconductor compositions or elements have been shown to the art which are capable of producing a significant improvement in substantially all of the aforementioned desirable characteristics. Conventional dye addenda to photoconductor compositions have generally shown only a limited capability for over-all improvement in the totality of electrophotographic properties which cooperate to produce a useful electrophotographic element or structure. The art is still searching for improvements in shoulder and toe speeds, improved solid area reproduction characteristics, rapid recovery and useful electrophotographic speed from either positive or negative electrostatic charging.

A high speed aggregate photoconductive system Was developed by William A. Light which overcomes many of the problems of the prior art. This aggregate composition is the subject matter of copending application Ser. No. 674,005, filed Oct. 9, 1967, now abandoned. However, there is a need for photoconductive elements of even higher electrophotographic speeds.

It is, therefore, an object of this invention to provide the art of electrophotography with novel photoconductive elements having improved electrophotographic speeds.

It is a further object of this invention to provide a novel method for preparing higher speed photoconductive elements.

It is another object of this invention to provide a novel means for increasing the electrophotographic speeds of aggregate photoconductive compositions.

It is likewise an object of this invention to provide novel photoconductive elements having high relative efficiency when electrically charged to a negative potential.

These and further objects and advantages of the invention will become apparent from the following description of the invention.

It has been discovered that when the heterogeneous or aggregate photoconductive compositions of William A. Light are overcoated in a certain manner, a large increase in electrophotographic speed can be effected. In particular, when such aggregate compositions are overcoated with one or more coatings of a solution of a sensitiping dye. of the type useful in the aggregate composition, the electrophotographic speed can be substantially increased.

The aggregate photoconductive compositions are in general composed of a photoconductor sensitized with a sensitizing composition comprising a two-phase heterogeneous combination of a, sensitizing dye and a film-forming hydrophobic polymer. Many dyes and mixtures of dyes, such as pyrylium dyes, including pyrylium, selena:

pyrylium and thiapyrylium dye salts, are useful in forming these heterogeneous compositions. These heterogenous compositions are formed by combining the sensitizing dye with the hydrophobic polymer under conditions which result in the formation of a separately identifiable two-phase state having a radiation absorption maximum that is substantially shifted from the radiation maximum characteristic of the dye simply dissolved in the polymer.

The aggregate photoconductive compositions can be prepared in a variety of ways. A solution containing the constituents of the aggregate photoconductive compositions can be coated in the form of a layer in a conventional manner onto a suitable support and the formation of the aggregate composition of the invention achieved in situ in the formed layer. One technique for converting a homogeneous coating of dye and polymer to the heterogeneous system is by prolonged contact of the coating to vapors of solvent which are capable of softening the layer, the dye being caused to migrate and form aggregates in a two phase system. Usually such vapor exposure is effective to permit formation of a substantial amount of the heterogeneons compositions from the dye and polymer in about two minutes at about 70 F. Likewise, inhibition of solvent removal in an otherwise normal coating operation of a dope solution made up of the dye and polymer can form the heterogeneous compositions. Similarly, immersing the homogeneous coating in a solvent, or coating from an original solvent mixture which contains a high boiling solvent which persists in the coating during drying, are among other methods of forming the heterogeneous compositions. Still another useful method of forming the aggregate photoconductive compositions is by vigorous, high speed shearing of the constituents, coating of the sheared solution, and drying.

The compositions when formed in situ in a photoconductive layer have an identifiable heterogeneous appearance when viewed under 2500 magnification, although such compositions may appear to be substantially optically clear to the naked eye in the absence of magnification. The aggregate compositions can, however, exhibit a macroscopic heterogeneity. Typically the aggregate in the discontinuous phase is predominantly in the size range of about .1 to microns.

Sensitizing dyes and hydrophobic polymeric materials are used in forming these heterogeneous compositions. Typically, pyrylium dyes, including pyrylium, thiapyrylium and selenapyrylium dye salts are useful in forming such compositions. Such dyes include those which can be represented by the following general formula:

Rb a

wherein R R R, R and R can each represent a hydrogen atom; an aliphatic or aromatic group typically having from 1 to 15 carbon atoms, such as alkyl groups, methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, amyl, isoamyl, hexyl, octyl, onyl, dodecyl, styryl, methoxystyryl, diethoxystyryl, dimethlaminostyryl, l-butyl- 4 p dimethylaminophenyl-1,3-bntadienyl, ,c-ethyl-4-dimethylaminostyryl; alkoxy groups like methoxy, ethoxy, propoxy, butoxy, amyloxy, hexoxy, octoxy, and the like; aryl groups like phenyl, 4-diphenyl, alkylphenyls as 4- ethyl-phenyl, 4-propylphenyl, and the like, alkoxyphenyls as 4-ethoxyphenyl, 4-methoxyphenyl, 4-amyloxyphenyl, 2- hexoxyphenyl, Z-methoxyphenyl, 3,4-dimethoxyphenyl, and the like, fl-hydroxyalkoxyphcnyls as Z-hydroxyethoxyphenyl, B-hydroxyethoxyphenyl, and the like, 4-hydroxy phenyl, halophenyls as 2,4-dichlorophenyl, 3,4-dibromophenyl, 4-chlorophenyl, 2,4-dichlorophenyl, and the like, azidophenyl, nitrophenyl, aminophenyls as 4-diethylaminophenyl, 4-dimethylaminophenyl and the like, and naphthyl; vinyl, and the like; and where X is a sulfur,

oxygen or selenium atom, and Z is an anionic function, including such anions as perchlorate, fluoroborate, iodide, chloride, bromide, sulfate, sulfonate, periodate, p-toluenesulfonate, hexafluorophosphate, and the like. In addition, the pair R and R as well as the pair R and R can together be the necessary atoms to complete an aryl ring fused to the pyrylium nucleus. Typical members of such pyrylium dyes are listed in Table 1.

10 TABLE 1 Compound N 0.

Name of compound 1 4-(4bis-(2-chloroethyl)aminophenyl)-2,6-diphenylthiapyrylium perchlorate.

2,4,6-triphenylpyrylium perchlorate.

4-(4-methoxyphenyl)-2,6-diphenylpyrylium perchlorate. 4-(2,4-dichlorophenyl)-2,6-diphenylpyrylium perchlorate. 4-(3,4-dichlorophonyl)-2.6-diphenylpyrylium perchlorate. 2,6-bis(-methoxyphenyl)-4-phenylpyrylium perchlorate. 6-(4-methoxyphenyl)-2,4-diphenylpyrylimn perchlorate.

21 2-(3,4-dichlorophenyD-4-(4-methoxyphenyl)-6-phcnylpyrylium perchlorate. 22 4- (4-amyloxyphenyl)-2,6-bis i-ethylph enyl) pyrylium perchlorate. 23 4-(4-amyloxyphenyl)-2,6 bis(4-methoxypheny1) pyrylium perchlorate. 24 2,4,6-triphenylpyrylium fluoroboratc.

25 2,6-bis (4-cthylphenyl) --(4-methoxyphenyl) pyrylium perchlorate. 26 2,6-bis (4-cthylphenyl) -4-(4-methoxyphenyl) pyrylium iluoroborate. 27 6-(3,4diethoxystyryl)-2,4-d1phenylpyrylium perchlorate. 28 6-(3, t-diethoxy-B-amylstyryl)-2,4-diphenylpyrylium fluoroborate. v 20 6-( t-dimethylamlno-B-ethylstyryl)-2,4-d1phenylpyrylimn fluoroborate. 30 6-(l-n-amyl-et-p-dlmethvlamlnophcnyl-l,3-butadienyl)- 2,4-diphenylpyryhum fluoroborate. 31 6-(4-dimethylaminostyryl) -2,4diphenylpyrylium fluoroborate. 32 6-[a-etl1yl-B,B-b1s(duncthylammophenyl)vinylene]- 2,4diphcnylpyryhum fluoroborate.

33 6-(l-bntylt-p-dimethylaminophenyl-l,S-butadicnyl)- 2,4diphenylpyrylium fluoroborate. 34 6-( t'dimethylaminostyryl)-2,-t-cliphenylpyryllum perchlorate. 35 (i-[Bfl-bis(4-dimethylammophenyl)viny1ene]-2,4-diphenylpyrylium perchlorate. 36 2,6-bis(4-dimethylaminostyryl)-4-phenylpyrylium 5o perchlorate. I

37 6-(fl-methyl l-(limethylam1110styryl)-2,4-diphenylpyrylium fluoroborate. 38 6-(1-ethyl-4-p-dimethy1aminophenyl-1,3-butadien 1) 2,4-diphenylpyryhum fluoroborate. 39 fi-[fifi-bis( l-dimethylannnophenyl)vinylene]-2,4-diphenylpyrylium t'luoroborate. 4O 6-(l-methyl-4-p-dimethylaminophenyl-l-S-b utadienyD- 2,4-diphenylpyrylium Iluoroborate. 41 4-(4-dimeth ylaminoph enyl) -2,6-diph enylpyrylium perchlorate. 42 2,ti-bisM-ethylphenyl)-4phenylpyrylium perchlorate. 43- 2,6-bis( l-ethylphenyl)-4methoxyphenylthiapyrylimn fluoroborate. 4A. 2,4,6-triphenylthiapyryhum perchlorate. 45 4-(+methoxyphenyl)-2,G-diphenylthiapyrylium perchlorate. 46 6( lmethoxyphenyl)-2,4-dipltenylthiapyrylium perchlorate. 47 2,6-bis(4-meth oxyphenyl)-4-phenylthiapyrylium perchlorate. 48 4-(2,4-dichloropheuyl)-2,6-diphcnylthiapy'ryliuln 7 0 perchlorate.

49 2,4,6-tri(4-1nethoxyphenyl) thlapyryhum perchlorate. 50 2,6-bis ipthylphcnyl)-4phenylthinpyrylium perchlorate. 51 4(4amyloxyphenyl) 2,6-bis(4-ethylphenyl)thiapyrylium perchlorate. 5'2. fi-(tdimcthylaminostyt'yl)-2,ldiphcnylthiapyrylium perchlorate. 53 2,4,6-triphenylthiapyrylium fluoroborate. 54 2,4,6-triphenylthiapyryltum sulfate.

cluding copolymers, are those linear polymers having the wherein:

R and R when taken separately, can each be a hydrogen atom, an alkyl radical such as methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl and the like including substituted alkyl radicals such as trifluoromethyl etc., and an aryl radical such as phenyl and naphthyl including substituted aryl radicals having such substituents as a halogen, alkyl radicals of from 1 t carbon atoms, etc.; and R and R when taken together, can represent the carbon atoms necessary to form a cyclic hydrocarbon radical including cycloalkanes such as cyclohexyl and polycycloalkanes such as norbornyl, the total number of carbon atoms in R and R being up to 19;

R and R; can each be hydrogen, an alkyl radical of from 1 to 5 carbon atoms or a halogen such as chloro, bromo, iodo, etc.; and

R is a divalent radical selected from the following:

Among the diarylalkane polymers particularly useful in forming the aggregate compositions are polymers comprised of the following recurring unit:

wherein:

Each R is a phenylene radical including halo substituted phenylene radicals and alkyl substituted phenylene radicals; and R and R are described above. Such compositions are disclosed, for example, in US. Pat. Nos. 3,028,365 and 3,317,466.

Preferably polycarbonates containing diarylalkane moieties such as those prepared with Bisphenol A and including polymeric products of ester exchange between diphenylcarbonate and 2,2-bis-4-hydroxyphenyl propane are useful in the practice or" this invention. Such compositions are disclosed in the following US. Pats.: 2,999,750; 3,038,874; 3,038,879; 3,038,880; 3,106,544; 3,106,545; 3,106,546; and published Australian patent specification No. 19575/56.

Liquids useful for treating coatings to form the heterogeneous compositions can include water, a number of organic solvents such as aromatic hydrocarbons like benzene and toluene, ketones like acetone and ethylmethyl ketone, halogenated hydrocarbons, like methylene chloride and ethylene chloride, ethers like tetrahydrofuran, alkyl and aryl alcohols like methyl, ethyl, and benzyl alcohol, as well as mixtures of such solvents.

A wide range of photoconductors can be used in the aggregate photoconductive elements prepared above. These elements can be prepared with an organic, including organometallic, photoconducting material which has little or substantially no persistence of photoconductivity. Representative organo-metallic compounds are the organic derivatives of Group Na and Va metals such as those having at least one amino-aryl group attached to the metal atom. Exemplary organo-metallic compounds are the triphenyl-p-dialkylaminophenyl derivatives of silicon, germanium, tin and lead and the tri-p-dialkylaminophenyl derivatives of arsenic, antimony, phosphorus, and bismuth.

An especially useful class of organic photoconductors is referred to herein as organic amine photoconductors. Such organic photoconductors have as a common structural feature at least one amino group. Useful organic photoconductors include arylamine compounds such as (1) diarylamines like diphenylamine; dinaphthylamine; N,N' diphenylbenzidine; N-phenyl-l-naphthylamine; N- phenyl 2 naphthylamine; N,N diphenyl-p-phenylenediamine; 2-carboxy-5-chloro-4'-methoxydiphenylamine; panilinophenol; N,N-di 2 naphthyl-p-phenylenediamine, and the like, and (2) triarylamines including (a) nonpolymeric triarylamines, like triphenylamine, N,N,N,N'- tetraphenyl m phenylenediamine; acetyltriphenylamine; lauroyltriphenylarnine; hexyltriphenylamine; dodecyltriphenylamine; hexaphenylpararosaniline; 4,4-bis( diphenylamino)benzil; 4,4'-'bis(diphenylamino)benzophenone and the like, and (b) polymeric triarylamines like poly-[N,4"- N,N',N-triphenylbenzidine) polyadipyltriphenylamine; polysebacyltriphenylamine; polydecamethylenetriphenylamine; poly N (4-vinylphenyl)-diphenylamine; poly-N- (vinylphenyl)-tx,ot'-dinaphthylamine and the like.

Suitable organic amine photoconductors can be represented by polyarylalkane compounds having the formula:

HLE

wherein each of D, E and G is an aryl radical, J is selected from the group consisting of a hydrogen atom, an alkyl radical and an aryl radical, at least one of D, E and G containing an amino substituent, such as the amino-substituted triphenylmethane leuco bases disclosed in French Pat. No. 1,383,461.

Representative photoconductive compounds useful with sensitizing amounts of the above heterogeneous compositions include the following:

TAB LE 3 Compound No. Name 01 compound 3 4 ,4 -bis (diethylamino) -2,6-diCh1oro-2 ,2 -din1ethyltriphenylmethane.

4 4,4-bis (diethylarnino) -2,2dimethyldiphenylnaphthylmethane.

5 2 ,2 -dimethyl-4,4 ,4 -tris(dimetliyl amino) triphenylmethane.

6 4,4-bis (diethylamiuo) 2-tlimethylamino-2,2,5,5-

tetramethyltripbenylmethane.

7 4,4-bis(diethylamino)-2-chloro-2,2-dimethyl-4-dim thylaminotriphenylmethane.

8 l 4,4-bis (diethylamino) A-dimethylamino-2,2,2-trimethyltriphenylmethane.

9 4.4-bis (dimethylamino) -2-chl0ro-2,2-dimethyltriphenylmethaue.

10 4 ,4 -bis(dimethylamino) -2 ,2 -din1ethyl-4-methoxy triphcnylmethane.

11 4,4-bis(benzylethylamino)-2,2-dimethyltriphenylmethane.

12 4 ,4 -bis (diethylamino) -2 ,2 ,5 ,5 -tetramethyltriphenylmethane.

13 4 ,4-bis (diethylamino) -2,2 -diethoxytriphenylmethane.

The following Table 4 comprises a partial listing of US. patents disclosing a wide variety of organic photoconductive compounds and compositions which are also useful.

Table 4 Inventor: U.S. Pat. No. Hoegl et a1 3,037,861 Sues et al. 3,041,165 Schlesinger 3,066,023 Bethe 3,072,479 Klupfel et al. 3,047,095 Neugebauer et al. 3,112,197 Cassiers et a1. 3,133,022 Schlesinger 3,144,633 Noe et al. 3,122,435

Inventor: US. Pat. No.

Sues et al. 3,127,266 Schlesinger 3,130,046 Cassiers 3,131,060 Schlesinger 3,139,338 Schlesinger 3,139,339 Cassiers 3,140,946 Davis et al. 3,141,770 Ghys 3,148,982 Cassiers 3,155,503 Cassiers 3,131,060 Cassiers 3,158,475 Tomanek 3,161,505 Schlesinger 3,163,530 Schlesinger 3,163,531 Schlesinger 3,163,532 Hoegl 3,169,060 Stumpf 3,174,854 Klupfel et al. 3,180,729 Klupfel et al. 3,180,730 Neugebauer 3,189,447 Neugebauer 3,206,306 Fox 3,240,597 Schlesinger 3,257,202 Sues et al 3,257,203 Sues et al. 3,257,204 Fox 3,265,496 Kosche 3. 3,265,497 Noe et al. 3,274,000

The electrophotographic speeds of such aggregate photoconductive compositions can be substantially 1ncreased in accordance with this invention by overcoat ng the compositions with at least one coating of a solution of a sensitizing dye of the type described above used 1n preparing the aggregate compositions. Many dyes and mixtures thereof, such as pyrylium dyes, including such pyrylium dyes as pyrylium, selenapyrylium and thrapyr y1 ium dye salts, listed above, are useful in the overcoating procedure of this invention. Although the dyes useful n the overcoating solution include any of the dyes that will form the so-called aggregate with a hydrophobic polymer, the dye used in a particular overcoat appl1cat1on need not be exactly the same as the dye in the aggregate base layer. Preferably, however, the dye used In the overcoat is comparable in structure to the dye in the aggregate photoconductive layer.

The solvents useful for preparing the overcoating solutions of this invention can be widely varied among polar hydrocarbon coating solvents. Preferred solvents are halogenated hydrocarbon coating solvents. The solvent, of course, must be one which will dissolve the dye chosen. In addition, the solvent must be such that it is capable of at least causing swelling of the polymer component of the aggregate photoconductive composition. The solvent must be volatile and preferably has a boiling polnt of less than 200 C. Particularly useful solvents include halogenated lower alkanes having from 1 to 3 carbon atoms, such as dichloromethane, dichloroethane, dichloropropane, trichloromethane, trichloroethane, tribromomethane, trichloromonofluoromethane, trichlorotrifiuoroethane, etc.; halogenated benzene compounds such as chlorobenzene, bromobenzene, dichlorobenzene, etc.; and the like.

The present invention is not limited to only one overcoating step. In fact, aggregate photoconductive elements overcoated a number of times with the dye solutions of this invention show electrophotographic speed increases with each new overcoat. However, from the point of practicality, more than about eight overcoats may not be justified by the decreasing effective speed increase attributable to an additional overcoat.

The concentration of dye in the overcoating solution can be varied considerably, being limited, of course, by

the solubility of a particular dye in a particular solvent. Higher concentrations of dye are preferred as low dye concentrations give only small speed increases. In addition, the dye solution can contain other materials in order to facilitate coating, such as a material to increase the solution viscosity. Polymeric materials, such as high viscosity carbonates prepared with Bisphenol A are just one example of materials that can be used to increase the solution viscosity. When coating these solutions, it has been found that the coating rate can be varied. However, coverage rates greater than about 3 ml./ft. are typically avoided to minimize mottling in the coating. Also, it has been noted that the coating rate is substantially independent of dye concentration or solution viscosity.

It has further been found that the total dye concentration in an overcoated aggregate composition of this invention is not the sole reason for the increased electrophotographic speeds. A typical comparison of an ordinary non-overcoated photoconductive element having an aggregate photoconductive composition coating containing dye in a concentration equal to the total dye concentration of an overcoated aggregate photoconductive composition of this invention shows the latter to be faster by a factor of four times the speed of the former. Such a comparison clearly shows that the speed increases obtained in accordance with the invention are not merely dependent upon dye concentration but, rather the speed increases are a result of the cooperating combination of an overcoat of sensitizing dye on an aggregated photoconductive composition.

The dye overcoats of this invention are imbibed into the photoconductive layer such that there is little or no dimensional change in the thickness of the layer after overcoating. Normally the base photoconductive layers, which are overcoated in accordance with this invention, range in thickness from about 5 to about 15,u.

Another feature of this invention is the fact that the dye in the overcoat solution changes its radiation absorption characteristics after being imbibed into the base photoconductive layer. The radiation absorption maximum of the dye or dyes after imbibition in accordance with this invention is substantially shifted from the radiation absorption maximum of the dye or dyes when merely in solution. Such an absorption maximum shift after imbibition is generally of the magnitude of at least about 10 my" In general, the present invention is practiced by forming a layer of an aggregate photoconductive composition on a conducting support and then overcoating the photoconductive layer one or more times with a solution containing a sensitizing dye of the type referred to above. The dye solution is imbibed into the photoconductive layer and the solvent is evaporated. The resultant electrophotographic element can then be employed in any of the well known electrophotographic processes which require photoconductive layers.

One such process is the xerographic process. In a process of this type, and electrophotographic element is held in the dark and given a blanket electrostatic charge by placing it under a corona discharge. This uniform charge is retained by the layer because of the substantial dark insulating property of the layer, i.e., the low conductivity of the layer in the dark. The electrostatic charge formed on the surface of the photoconductive layer is then selectively dissipated from the surface of the layer by imagewise exposure to light by means of a conventional exposure operation such as, for example, by a con tact-printing technique, or by lens projection of an image, and the like, to thereby form a latent electrostatic image in the photoconductive layer. Exposing the surface in this manner forms a pattern of electrostatic charge by virtue of the fact that light energy striking the photoconductor causes the electrostatic charge in the light struck areas to be conducted away from the surface in proportion to the intensity of the illumination in a particular area.

The charge pattern produced by exposure is then developed or transferred to another surface and developed there, i.e., either the charged or uncharged areas rendered visible, by treatment with a medium comprising electrostatically responsive particles having optical density. The developing electrostatically responsive particles can be in the form of a dust, i.e., powder, a pigment in a resinous carrier, i.e., toner. A preferred method of applying such toner to a latent electrostatic image for solid area development is by the use of a magnetic brush. Methods of forming are using a magnetic brush toner applicator are described in the following U.S. Pats: 2,786,439; 2,786,440; 2,786,441; 2,811,465; 2,874,063; 2,984,163; 3,040,704; 3,117,884; and Reissue 25,779. Liquid development of the latent electrostatic image may also be used. In liquid development the developing particles are carried to the image-bearing surface in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature, for example, U.S. Pat. 2,907,674 and in Australian Pat. 212,315. In dry developing processes, the most widely used method of obtaining a permanent record is achieved by selecting a developing particle which has as one of its components a low-melting resin. Heating the powder image then causes the resin to melt or fuse into or on the element. The powder is, therefore, caused to adhere permanently to the surface of the photocnductive layer. In other cases, a transfer of the electrostatic charge image formed on the photoconductive layer can be made to a second support such as paper which would then become the final print after development and fusing. Techniques of the type indicated are well known in the art and have been described in a number of U.S. and foreign patents, such as U.S. Pat. 2,297,691 and 2,551,582 and in RCA Review Vol. 15 (1954) pages 469-484.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 An aggregate photoconductive composition is prepared in accordance with the method disclosed in E. P. Gramza U.S. application Ser. No. 674,006, filed Oct. 9, 1967, now abandoned, by vigorous, high speed shearing of a solution having a solids content of about of a mixture of approximately 3% by weight of 4-(4-dimethylaminophenyl)-2,6-diphenylthiapyrylium perchlorate sensitizing dye salt, 39% of 4,4-diethylamine-2,2-dimethyltriphenylmethane photoconductor and 58% of a polycarbonate resin formed from the reaction between phosgene and a dihydroxydiarylalkane or from the ester exchange between diphenylcarbonate and 2,2-bis-4-hydroxyphenyl propane (Lexan 105 polycarbonate resin, General Electric Company). This composition is then wet-coated onto a conducting support which comprises high vacuum evap orated nickel film coated on a poly(ethylene terephthalate) film base which is subbed with a terpolymer of 2% by weight itaconic acid, methylacrylate and 83% vinylidene chloride. The coating is allowed to dry and when dry has a thickness of about 10 The resultant electrophotographic element is then electrostatically charged under a corona source until the surface potential, as measured by an electrometer probe, reaches about 600 volts. The charged element is then exposed to a 3000 K. tungsten light source through a stepped density gray scale. All exposures are also through a short wavelength pass interference filter having transmittance at 600 m The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial potential, V0, to some lower potential, V, whose exact value depends on the actual amount of exposure in meter-candle-seconds received by the area. The results of these meausrements are then plotted on a graph of surface potential V vs. log exposure for each step. The actual positive or negative speed of the photoconductive composition can then be expressed in terms of the reciprocal of the exposure required to reduce the surface potential to any fixed arbitrarily selected value. Herein, unless otherwise stated, the actual positive or negative speed is the numerical expression of 10 divided by the exposure in meter-candle-seconds required to reduce the 600 volt charged surface potential to a value of 50 volts. Speeds thus determined are referred to as positive or negative 50 volt toe speeds. The speed of the non-overcoated element is recorded in Table 5 below. Next, the photoconductive composition on the above element is overcoated from an extrusion hopper with a 0.45% solution of 4-(4 dimethylaminophenyl)-2,6-diphenylthiapyrylium perchlorate in dichloromethane. The dye solution is coated at a rate of 2 ml./ft. /pass which is equal to 12 mg. dye/ft /pass. The speed is then measured in accordance with the above procedure. Next, the element is overcoated a second and third time measuring the speeds after each overcoat. The speeds are shown in the table below.

The above comparison shows that increases in speed can be obtained with each new coating. Similar results are obtained using 1,1,2-trichloroethane, mixtures of methylene chloride and trichloroethane or using bromoform as the solvent in the dye overcoat. In addition, similar results are obtained when the dye in the overcoat solution is not the same as the dye in the aggregate photoconductive composition. For example, when 4-(4-dimethylaminophenyl)-2,6-diphenylthiapyrylium iluoroborate is used in solution to overcoat an element identical to the one above, similar increases in speed are seen with each overcoat. The charged and exposed electrophotographic elements described can be developed to form visible images with liquid developers of the type described in U.S. Pat. 2,907,674.

EXAMPLE 2 A first electrophotographic element with no overcoat is prepared in accordance with the procedure of Example 1, only 10 parts of the sheared photoconductive composition are added to parts of the unsheared composition followed by a brief period of shearing. The resultant composition is then coated as in Example 1. Next, a solution of 0.45 weight percent of 4-(4-dimethylaminophenyl)- 2,6-diphenylthiapyrylium perchlorate in methylene chloride is coated over the photoconductive layer of the element. The solution is coated at a rate of 2 cc./ft. per pass. The photoconductive layer is given three of these overcoats. The overcoated element is then measured for optical density. The absolute value for the optical density of the overcoated photoconductive layer is shown in Table 6. The absolute value is the optical density of the completed element less the value (0.5) of the nickel-coated substrate. The completed element is then charged and exposed in the manner of Example 1. The 50 volt negative toe speed is shown in the table below. Next, a second element is prepared having a dye concentration, as measured by the optical density of the photoconductive layer, similar to that of the first element above. The photoconductive composition for the second element contains 55% by weight of a polymeric mixture of 50% by weight of the polycarbonate resin of Example 1 and 50% by weight of high viscosity polycarbonate prepared with Bisphenol A and having a viscosity of 2.70 centipoises, 40% of 4,4- diethylamino-2,2' dimethyltriphenylmethane photoconductor and 5% of 4 (4-dimethylaminophenyl)-2,6-diphenylthiapyrylium perch orate sensitizing dye salt. This composition is then subjected to vigorous, high-speed shearing and then used to prepare an electrophotographic TAB LE 6 Absolute optical density Speed -50 v. at 600 m (toe speed) Second element, no overcoat 1. 43 450 First element, overcoated 1. 40 1, 800

This comparison shows that the increase in electrophoto graphic speed is not simply a function of dye concentration as the second element has a slightly greater dye concentration but is four times slower than the first element.

EXAMPLE 3 A non-overcoated electrophotographic element is prepared as in Example 1 and used as a control. Next, a second element is prepared similar to the first and overcoated as in Example 1 using only the dye solvent, methylene chloride, with no dye present. Finally, a third electrophotographic element is prepared as above and overcoated with a solution of about 0.4 weight percent of 4- (4-dimethylaminophenyl) 2,6 diphenylthiapyrylum perchlorate in methylene chloride. Both are second and third elements are overcoated at a rate of 2 ml./ft. Next, all three electrophotographic elements are measured for electrical speed in accordance with the procedure of Example 1. The positive and negative speeds are shown in Table 7 below.

TABLE 7 Electrical speed 50 toe 50 too First element (no overcoat) 900 1, 000 Second element (solvent overcoat) 630 900 Third element (solvent plus dye overcoat) 1, 200 1, 800

This example shows that the coating solvent itself does not contribute to the speed increase of a fully aggregated photoconductive composition.

EXAMPLE 4 Example 1 is repeated using bis(4-diethylamino)-1,l,1- triphenylethane in place of the 4,4-diethy1arnino-2,2'-dimethyltriphenylmethane as the photoconductor. The resultant non-overcoated electrophotographic element is measured for electrical speed and is then overcoated as in Example 1 with two coatings of the dye solution and measured again for electrical speed. Optical density and electrical speed measurements are shown in Table 8 below.

Example 1 is repeated with the aggregate photoconductive composition being coated without an overcoat to produce a layer thickness of 12.5 m dry. The resultant electrophotographic element is then measured for total optical density and electrical speed. Next, the element is given one overcoat of dye as in Example 1 and measured again for density and speed. The element is then overcoated two more times and measured for optical 14 density and electrical speed. The results of these measurements are shown in Table 9 below.

TAB LE 9 Total optical density Speed -50 v. at 600 m (toe speed) Control (no overcoat). 1. 25 1,100 1 overcoat 2.00 1,800 3 overcoats 2. 2, 600

EXAMPLE 6 A non-overcoated electrophotographic element is pre pared and measured as in Example 1. Next, the element is given an overcoat of a 1.25% solution of 4-(4-dimethylaminophenyl) 2,6 diphenylthiapyrylium fiuoroborate in methylene chloride. The dye solution is coated at a rate of 1.5 cc./ft. After allowing the overcoat to dry, the element is measured for optical density and electrical speed. Table 10 shows the density and speed measurements.

pared in accordance with the procedure of Example 1 using 4 (4-dimethylaminophenyl)-2,6-diphenylthiapyrylium fiuoroborate as the dye in the aggregated photoconductive composition. After drying, the photoconductive layer is overcoated with two coatings of the dye overcoat solution of Example 1. After final drying the element is charged as previously described and imagewise exposed. The latent electrostatic image is then developed by cascade development, as described in U.S. Pat. No. 2,618,552, using a developer mix composed of glass beads and a toner powder of polystyrene and carbon black. This mix is cascaded across the surface of the photoconductive layer and the electroscopic toner powder adheres to the charged areas of the latent electrostatic image. The resultant developed image is then transferred to a receiving sheet and the powder image is fused by heating. A good image results.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A method of increasing the electrophotographic speed of a heterogeneous photoconductive composition comprising an organic photoconductor and a two phase combination of a pyrylium dye and a polymer having the following recurring unit:

Re 1'14 R1 1 -C $Q L 5 I wherein: each of R and R when taken separately, is selected from R is selected from the group consisting of divalent radicals having the formulae:

said two phases being visible under 25 O magnification and the radiation absorption maximum for said two-phase composition being substantially shifted from the absorption maximum for a homogeneous combination of said photoconductor, dye and polymer, said method comprising the steps of overcoating said composition with at least one coating of a solution of a pyrylium dye whereby the dye is imbibed into the photoconductive composition to combine with said polymer, the maximum radiation absorption of said overcoated composition being different from the absorption maximum of the dye of the overcoat ing solution.

2. A process as in claim 1 wherein the sensitizing dye is 4 (4-dimethylaminophenyl) 2,6 diphenylthiapyrylium perchlorate.

3. A method as described in claim 1 wherein the solution of pyrylium dye is comprised of a polar hydrocarbon solvent containing a member selected from the group consisting of thiapyrylium, pyrylium and selenapyrylium dye salts.

4. A process as in claim 3 wherein the solvent is selected from the group consisting of a halogenated lower alkane having from 1 to 3 carbon atoms, a halogenated benzene and a halogenated ether having up to 5 carbon atoms.

5. A process as in claim 4 wherein the solvent is selected from the group consisting of dichloromethane', dichloroethane, dichloropropane, trichloromethane, trichloroethane, tribromomethane, trichloromonofluoromethane and trichlorotrifiuoroethane.

6. In a electrophotographic element comprising an electrically conductive support having coated thereon a photoconductive composition comprising an organic photoconductor sensitized with a two-phase heterogeneous combination of a pyrylium dye and a film-forming polycarbonate resin, said composition having a radiation absorption maximum that is substantially shifted from the radiation absorption maximum of the dye dissolved in the resin, the

1 6 improvement wherein the electrophotographic speed of said photoconductive layer is increased by the method of claim 1.

7. A photoconductive element as in claim 6 wherein the dye in the overcoating solution is a thiapyrylium dye salt.

8. A photoconductive element as in claim 6 wherein the dye in the overcoating solution is a selenapyrylium dye salt.

9. A photoconductive element as in claim 6 wherein the dye in the overcoating solution is a pyrylium dye salt.

10. A photoconductive element as in claim 6 wherein the dye in the overcoating solution is 4-(4-dimethylaminophenyl) -2,6-diphenylthiapyrylium perchlorate.

11. A photoconductive element as in claim 6 wherein the dye in the overcoating solution is 4-(4-dimethylaminophenyl)-2,6-diphenylthiapyrylium tluoroborate.

12. A photoconductive element as in claim 6 wherein the organic photoconductor is selected from the group consisting of a polyarylalkane having the formula:

wherein each of D, E and G is an aryl radical, J is selected from the group consisting of a hydrogen atom, an alkyl radical and an aryl radical, at least one of D, E, and G containing an amino substituent; a Group IVa organometallic compound having at least one aminoaryl radical attached to a Group IVa metal; and a Group Va organometallic compound having at least one aminoaryl radical attached to a Group Va metal.

References Cited UNITED STATES PATENTS 3,121,008 2/1964 Jones et al. 96-1 3,141,770 7/1964 Darus et a1 96-1 3,250,615 5/ 1966 Van Allen et al 961 3,251,687 5/1966 Fohl et a1. 961 3,394,001 7/1968 Makeno 961.5

GEORGE F. LESMES, Primary Examiner J. C. COOPER III, Assistant Examiner US. Cl. X.R. 961;252501 

